Continuous polymerization of conjugated dienes in the production of homopolymers and copolymers by organolithium initiation in solution polymerization systems offers real production advantages, such as more economic production and more uniform compositions. Process variables can be readily adjusted in a continuous operation so as to optimize operations for a given polymerization recipe. Minor adjustments of monomer feed rates, initiator rates, randomizer rates, temperature control, pressure control, solvent usage, all can be readily made in a continuous process, in effect "fine tuning" so as to maintain relatively consistently a product with optimum properties.
Unfortunately, continuous solution polymerization has the disadvantage of a tendency toward fouling, especially in the first reactor means of the reactor system through a buildup of polymeric materials on the surfaces of the reactor means in contact with the reactants, on heat transfer surfaces, stirring or circulation means, and the like, often ending prematurely a continuous polymerization run. The longer than equipment can be operated on a continuous basis, the more economical the overall results. Having to stop the entire process, clean the equipment, and then start up all over again, means attendant startup problems and necessity to again fine tune the operation. Gel, of course, adversely affects polymer uniformity and quality.
Methods of reducing reactor fouling generally involve the addition of a gel inhibitor to the polymerization system. Gel inhibitors act as slow polymerization poisons which reduce the number of living polymer chains tending to grow and accumulate on the interior surfaces of the polymerization reactor and which otherwise would form high molecular weight gel particles.
Such a gel-inhibitory mechanism, although effective in its intended purpose of inhibiting gel formation, also reduces the total number of nonterminated, live polymer chains otherwise ultimately available at the end of monomer conversion for coupling purposes.
Effective coupling of living polymer chains to form linear or radially branched polymer is important in obtaining polymers having higher molecular weight, broader molecular weight distribution, higher Mooney viscosity, and reduced cold flow, among other aspects.
What is needed is a continuous polymerization process, free of gel, yet providing efficient coupling.